Iron

Chemical reactions

Reaction of iron with acids


Iron metal dissolves readily in dilute sulphuric acid in the absence of oxygen forming Fe(II) ions and H2. In aqueous solution Fe(II) is present as the complex [Fe(H2O)6]2+.

Fe(s) + H2SO4(aq) Fe2+(aq) + SO42−(aq) + H2(g)

If oxygen is present, some of the Fe(II) oxidizes to Fe(III).

3 Fe(s) + 4 H2SO4(aq) Fe2+(aq) + 2 Fe3+(aq) + 4 SO42−(aq) + 4 H2(g)

Concentrated nitric acid, HNO3, reacts on the surface of iron and passivates the surface.


Reaction of iron with air


Iron reacts with oxygen, O2, forming Fe(II) and Fe(III) oxides. The oxide layer do not passivate the surface. Finely divided iron, e.g. powder or iron wool, can burn:

4 Fe(s) + 3 O2(g) 2 Fe2O3(s)
3 Fe(s) + 2 O2(g) Fe3O4(s)


Reaction of iron with ammonia


Fe(II) and Fe(III) does not form complexes with ammonia but are both precipitated as hydroxides:

NH3(aq) + H2O(l) NH4+(aq) + OH−(aq)
Fe2+(aq) + 2 OH−(aq) Fe(OH)2(s) [white]
Fe3+(aq) + 3 OH−(aq) Fe(OH)3(s) [reddish brown]


Reaction of iron with carbonate


Fe(II) is precipitated by carbonate ions:

Fe2+(aq) + CO32−(aq) FeCO3(s) [white]

The carbonate is readily oxidized to Fe(OH)3 when exposed to O2(g).

Fe(III) is not precipitated by carbonate ions, but will precipitate as the hydroxide:

CO32−(aq) + H2O(l) HCO3−(aq) + OH−(aq)
Fe3+(aq) + 3OH−(aq) Fe(OH)3(s) [reddish brown]


Reaction of iron with halogens


Iron reacts with excess of the halogens F2, Cl2, and Br2, to form Fe(III) halides.

2 Fe(s) + 3 F2(g) 2 FeF3(s) [white]
2 Fe(s) + 3 Cl2(g) 2 FeCl3(s) [dark brown]
2 Fe(s) + 3 Br2(l) 2 FeBr3(s) [reddish brown]

For I2, Fe is only oxidized to Fe(II)

Fe(s) + I2(s) FeI2(s) (grey)

Fe(III) forms yellow chloro complexes. The complexed are readily hydrolyzed by heat.

Fe(III) is reduced to Fe(II) by I−:

2 Fe3+(aq) + 2 I−(aq) 2 Fe2+(aq) + I2(aq)


Reaction of iron with hydroxide ions


Fe(II) is precipitated by hydroxide ions:

Fe2+(aq) + 2 OH−(aq) Fe(OH)2(s) [white]

Fe(III) is precipitated by hydroxide ions:

Fe3+(aq) + 3 OH−(aq) Fe(OH)3(s) [reddish brown]


Reaction of iron with phosphate


Fe(II) is not precipitated by phosphate ions in acetic acid.

Fe(III) is precipitated by phosphate ions in acetic acid:

Fe3+(aq) + H2PO4−(aq) FePO4(s) [light yellow] + 2 H+(aq)


Reaction of iron with hexacyanoferrate


Fe(II) is precipitated by hexacyanoferrate(II)ions. Under acidic conditions, the precipitate is white, however.

Fe2+(aq) + 2 K+(aq) + [Fe(CN)6]4−(aq) K2Fe[Fe(CN)6](s) [light blue]

Fe(III) is precipitated by hexacyanoferrate(II)ions under acidic conditions.

4 Fe3+(aq) + 3 [Fe(CN)6]4−(aq) Fe4[Fe(CN)6]3(s) [Turnbull's blue]

Fe(II) is precipitated by hexacyanoferrate(III)ions under acidic conditions:

21 Fe2+(aq) + 14 [Fe(CN)6]3−(aq) 5 Fe4[Fe(CN)6]3(s) [Turnbull's blue]

Turnbull's blue is destroyed by base:

Fe4[Fe(CN)6]3(s) + 12 OH−(aq) 4 Fe(OH)3(s) + 3 [Fe(CN)6]4−(aq)


Reaction of iron with sulfide


Fe(II) is not precipitated by sulfide under acidic conditions (i.e. H2S) but pricipitates with Na2S:

Fe2+(aq) + HS−(aq) + OH−(aq) FeS(s) [black] + H2O(l)

Fe(III) is not precipitated by sulfide under acidic conditions (i.e. H2S) but becomes reduced to Fe(II):

Fe3+(aq) + H2S(aq) Fe2+(aq) + S(s) + 2 H+(aq)


Reaction of iron with thiocyanate


Fe(II) forms colorless complexes with SCN−

Fe(III) forms red complexes with SCN−:

Fe3+(aq) + 3 SCN−(aq) + 3 H2O(l) Fe(SCN)3(H2O)3(aq)


Reaction of iron with water


Iron(III) reacts with water [1]:

Fe3+(aq) + 4 H2O(l) FeO42−(aq) + 8 H+(aq) + 3e−, E° = -2.20 V


Quantitative analysis


Method 3500-Fe C Inductively Coupled Plasma Method [2]. A portion of the sample is digested in a combination of acids. The digest is aspirated into an 8,000 K argon plasma where resulting light emission is quantified for 30 elements simultaneously.

Method limit of detection in water = 0.01 mg/L
Method limit of detection in soil = 100 mg/kg